Laser diodes are used extensively in the human society particularly in the field of optical telecommunication and optical storage of information. Further, various application of laser diodes are studied for example in the field of optical information processing such as optical computers.
In laser diodes, it is well known that the operational characteristics, particularly the threshold of laser oscillation, change with environmental temperature. Thus, in order to compensate for such a temperature-induced variation of the operational characteristics, the laser diode used for long range telecommunication purposes such as the devices used for optical submarine cables, use a temperature regulation system such that the laser diode is held at a constant temperature.
On the other hand, there exists other applications of laser diode wherein use of such a temperature regulation system is not possible or preferable. For example, the laser diodes used in the optical wiring of supercomputers have to be accommodated in a very limited space and the room for the temperature regulator is generally not available. Similarly, temperature regulation is not practical for the laser diodes that are used for the local area network (LAN), laser printers, and the like, because of the increased cost.
In order to avoid the unwanted temperature-induced fluctuation in the operation of laser diodes, conventional semiconductor optical sources use a feedback control of optical beam called automatic power control (APC) wherein the bias current is controlled such that the power of the output optical pulse of the laser diode is held constant. On the other hand, such an APC control has a problem in that, associated with the temperature-induced variation of the threshold of laser oscillation, the extinction ratio of the laser diode is deteriorated with increasing temperature.
FIG. 1 shows the conventional APC control applied to laser diodes, wherein the horizontal axis represents the drive current while the vertical axis represents the corresponding output optical power. Further, the line designated as P.sub.L represents the output characteristics of the laser diode at a low temperature, while the line designated as P.sub.H represents the output characteristics of the same laser diode at a high temperature.
Referring to FIG. 1, the drive current is supplied to the laser diode in the form of a current pulse i.sub.d, wherein the current pulse i.sub.d is biased to a threshold level i.sub.thL that represents the threshold level of the laser diode at the low temperature, such that the laser oscillation occurs with a minimum, threshold power level when no current pulse i.sub.d is supplied. Further, the magnitude of the current pulse i.sub.d is set such that a predetermined output power is achieved in response to the drive current pulse i.sub.d at the foregoing low temperature. In the illustrated example, the magnitude of the current pulse i.sub.d is set to 5 mA.
In the APC control, the biasing of the current pulse i.sub.d is subjected to a feedback control that is achieved in response to the output power of the laser diode, such that a predetermined output power such as 0.5 mW is secured even when the temperature of the laser diode changes. More specifically, the bias current added to the current pulse i.sub.d is changed such that the laser diode produces the foregoing predetermined output power in response to the current pulse i.sub.d.
When the biasing of the drive current pulse i.sub.d is set as such, there occurs a problem, when the temperature rises, in that the laser diode produces the optical output with a substantial power even when the current pulse i.sub.d is not supplied. In the illustrated example, the current pulse i.sub.d is biased at the level of 20 mA in the illustrated example, and the laser diode produces the output optical power of about 0.25 mW in the absence of the input current pulse i.sub.d. Thereby, the extinction ratio of the output optical beam is inevitably deteriorated. Associated with the degradation of the extinction ratio, discrimination of the data "0" and the data "1" at the reception side of the optical communication path becomes difficult.
In order to maintain the output power of the optical beam constant irrespective of variation of the temperature, it is also possible to change the magnitude of the drive current pulse in response to the variation of temperature. For example, one may fix the level of the bias current at i.sub.thl and increase the magnitude of the current pulse i.sub.d with temperature such that the output power of 0.5 mW is secured even when the temperature increases. According to this approach, one can maintain a large extinction ratio. However, the foregoing approach has a drawback in that there tends to occur a delay in the timing of the optical pulse in correspondence to the interval necessary for the carrier density in the active layer of the laser diode to increase to a sufficient level in response to the increase of the drive current from the level i.sub.thl to the desired level such as 25 mA. In other words, there occurs a time lag in the optical pulse with respect to the timing of the drive current pulse.